Stem cells are defined as clonogenic, self-renewing progenitor cells that can generate a wide variety of more specialised cell types via the process of differentiation. Classically, it has been believed that there are two distinct types of stem cell. Embryonic stem (ES) cells are derived from the inner mass of the blastocyst, are pluripotent and thus are capable of generating into all differentiated cell types within the body. The other sub-population of stem cells are derived from ES cells and are organ-or tissue-specific. These multipotent cells, also known as adult stem cells, were believed to be able to differentiate only into tissues from their organ of origin. An example of these multipotent cells are haematopoietic stem cells, which serve to continually regenerate the cells of the blood and immune system.
Stem cells have been isolated from a wide range of tissues, from those that have a high rate of ongoing cellular turnover, such as blood, cord blood, bone marrow, skin, intestine, and breast tissue, to those with a low turnover such as brain, skeletal muscle, and juvenile teeth. Irrespective of the tissue of origin, a long standing dogma has been that adult stem cells can only differentiate into the tissues from which they were derived. However recent work has demonstrated that upon exposure to a novel environment, organ-specific stem cells can overcome these intrinsic restrictions to transdifferentiate into other tissues. For example it has been shown that neural stem cells can transdifferentiate into blood cells, bone-marrow derived stem cells can transdifferentiate into muscle, brain, liver and heart cells, and skin derived stem cells can transdifferentiate into brain cells. Therefore it now appears likely that the dogma associated with developmental restriction of organ-specific stem cells is incorrect and it is feasible that these ES cells under appropriate enviromnental stimuli, can transdifferentiate into another cell type.
Human milk contains a mixture of different cell types. Secretory epithelial cells (lactocytes) are found in milk due to them sloughing off the basement membrane of the breast as a consequence of the pressure associated with the continued filling and emptying of the breast. Lactocytes account for approximately 10-20% of the total cell population. The majority of the remainder of cells found in human milk are leukocytes (immune cells such as lymphocytes, macrophages, monocytes, natural killer cells, basophils, eosinophils, and neutrophils), and are believed to be in milk to both protect the breast from infection and to provide immune protection for the infant. To date, these are the only cell types believed to be contained in milk.